| Résumé | Catechols and 1,8-naphthalene diols contain one “free” hydroxyl and one intramolecularly H-bonded hydroxyl group. The “free” hydroxyls are strong hydrogen-bond donors (HBDs) with α2H values (Abraham et al. J. Chem. Soc., PerkinTrans. 21989, 699) ranging from 0.685 to 0.775, indicating that these compounds have similar HBD properties to those of strongly acidic phenols such as 4-chlorophenol (α2H = 0.670) and 3, 5-dichlorophenol (α2H = 0.774). Kinetic effects on H-atom abstractions from the diols in HB acceptor (HBA) solvents can be quantitatively accounted for over at least 50% of the available range of solvent HBA activities (as measured by their β2H values; see Abraham et al. J. Chem. Soc. PerkinTrans. 21990, 521) on the basis of a single reactive OH group, the “free” OH. This free OH group is an outstanding H-atom donor in poor HBA solvents; e.g., in hexane rate constants for reaction with the DPPH• radical are 2.1 × 104 M-1 s-1 for 3,5-di-tert-butyl catechol and 2 × 106 M-1 s-1 for 4-methoxy-1,8-naphthalene diol, but only 7.4 × 103 M-1 s-1 for α-tocopherol (vitamin E). The diols are much more reactive than simple phenols because the O−H bond dissociation enthalpy of the “free” OH group is weakened by 5−9 kcal/mol by the intramolecular H-bond. The IR spectra of all the diols in CCl4 show two fairly sharp O−H stretching bands of roughly equal intensity separated by 42−138 cm-1. Addition of a low concentration of DMSO, a strong HBA, causes the band due to the intramolecularly H-bonded OH group to decrease in intensity to roughly half the extent that the “free” OH band loses intensity. The latter forms an intermolecular H-bond with the DMSO, the former does not. What has been overlooked in earlier work is that as the DMSO concentration is increased the band due to the intramolecularly H-bonded OH group first broadens and then evolves into a new, lower frequency (by 19−92 cm-1) band. The magnitude of the shift in the frequency of the intramolecular OH band caused by H-bonding of HBAs to the “free” OH group, Δν, increases linearly as the HBA activity of the additive increases, e.g., for 3,5-di-tert-butylcatechol, Δν/cm-1 = 33.8 β2H (R 2 = 0.986). This may provide a new and simple method for determining β2H values. |
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